Forensic Serology Questions and Answers

Answer: C

The red cell blood group systems are (b), (c) and (d) only: ABO, Kell and Duffy.

Red cell blood group systems are defined by antigens on the surface of erythrocytes. ABO, Kell and Duffy are all such systems recognised by the International Society of Blood Transfusion, and all have been used in forensic serology.

HLA (human leucocyte antigen) is the tissue-typing system found on white cells and most nucleated cells, not on red cells, so every option containing (a) is wrong.

Answer: A

The key gives bright blue to yellow green.

Luminol (the paper's 'luminal') is oxidised by hydrogen peroxide in an alkaline solution, with the iron of haemoglobin acting as a catalyst. The excited 3-aminophthalate formed emits light, seen in the dark as a bright blue glow; texts describe the emission as blue tending towards yellow-green. The glow lets investigators find traces of blood that have been washed or wiped away.

Luminol's chemiluminescence is never orange, violet-grey or white-green; those colours describe no reaction of the reagent with blood.

Answer: A

The amplification steps are (a), (b) and (c) only.

The polymerase chain reaction amplifies DNA in repeated cycles of three steps: denaturation, in which heat separates the two strands (b); annealing, in which primers bind to the ends of the target region on each strand (a); and extension, in which the polymerase adds individual nucleotides to build the new strand (c).

Southern blotting transfers DNA fragments from a gel to a membrane for probing. It belongs to the older RFLP method of DNA profiling, not to amplification, so options containing (d) are wrong.

Answer: B

The guaiacum test was used for blood.

The guaiacum test, one of the oldest presumptive tests for blood (nineteenth century), uses a resin from the guaiac tree with hydrogen peroxide or old turpentine. The peroxidase-like activity of haemoglobin oxidises the resin and turns it blue. It has been replaced by more sensitive reagents such as phenolphthalein (Kastle-Meyer), benzidine substitutes and luminol.

Saliva is detected by amylase, semen by acid phosphatase, and urine by urea or creatinine. None of these is tested with guaiacum.

Answer: A

Both statements are true.

A blood drop striking a surface at 90° leaves a round stain; at any smaller angle the stain is elongated into an ellipse, and the more acute the angle, the longer the stain. This is the basis of the bloodstain-pattern method for finding the angle of impact (Statement I).

The mathematical relationship of Statement II is

\[ \sin\alpha = \frac{\text{width}}{\text{length}} \]

so a stain 5 mm wide and 10 mm long struck at about 30°. Because both statements hold, the options calling either of them false are wrong.

Answer: C

The technique is competitive ELISA.

A sandwich ELISA captures the antigen between two antibodies, which needs at least two separate epitopes on the antigen. A small molecule (a drug, a hormone or a toxin) is too small to bind two antibodies at once and often has only one epitope.

In a competitive ELISA, the antigen in the sample competes with a labelled antigen for a limited number of antibody sites, so only one binding site is needed; a stronger sample means a weaker signal. That is why drug-screening immunoassays are competitive.

The double and triple antibody sandwich formats both need two epitopes, and DELFIA is a lanthanide-label detection system usually run as a sandwich, not the format defined by single-epitope antigens.

Answer: D

The optimum temperature for Taq polymerase is 72°C.

Taq DNA polymerase comes from Thermus aquaticus, a bacterium of hot springs. Its activity peaks at about 72-80°C, and it survives the repeated 94-95°C denaturation steps of PCR. That is why the extension step of each PCR cycle is run at 72°C.

About 54°C is a typical primer-annealing temperature, not the enzyme's optimum. 37°C is the optimum of enzymes from organisms such as E. coli, and 25°C is room temperature, at which Taq is barely active.

Answer: B

Star activity is the deviation of specificity of the cleavage site of a restriction enzyme.

Under non-optimal conditions, such as high glycerol, wrong salt concentration, wrong pH or very long digestion, a restriction enzyme begins to cut at sites that resemble, but do not exactly match, its recognition sequence. These extra cuts change the fragment pattern and can spoil an RFLP result.

Excess enzyme is one of the conditions that can cause star activity, but it is not what the term means. Separation by molecular weight is electrophoresis, and 'matching jump curve' is not a term associated with RFLP.

Answer: B

The method is nick translation.

In nick translation, DNase I introduces single-strand nicks into the DNA, and DNA polymerase I then removes nucleotides ahead of each nick while adding new ones behind it. If one of the nucleotides supplied is radioactively labelled (for example with ³²P), the nick moves along the strand and the probe becomes labelled. Such probes were used in RFLP Southern blotting.

Sanger sequencing reads a sequence; it is not a probe-labelling method. 'miRNA translation' and 'mtRNA translation' are not labelling techniques at all.

Answer: A

The Christmas tree stain identifies sperm cells.

The stain uses two dyes: nuclear fast red (Kernechtrot) and picroindigocarmine. Sperm heads stain red, with a paler pink acrosome, and the tails and necks stain green, while epithelial cells take on a blue-green colour. The red and green give the stain its name. It is the standard confirmatory microscopy for semen on swabs in sexual-assault cases.

Salivary and epithelial cells and red blood cells are not what the stain is designed to identify; they are identified by other tests.